The origins of the major animal body plans pose a challenging problem in evolutionary and developmental biology. There are about 35 recognized living animal phyla, each representing a distinct major body plan. These have shown an extraordinary evolutionary stability for over 500 million years. The developmental basis for transformations between body plans and for origins of body plans from more primitive evolutionary precursors are not known. The radial body plan of echinoderms has evolved from the bilaterally symmetrical body plan that their ancestors shared with the ancestors of the vertebrates. The circular sea urchin nervous system exemplifies this transformation. We will investigate changes in gene action involved in evolution of a radial CNS by use of a direct-developing species, whose large embryos develop into the adult morphology by 4 days after fertilization, and provide a unique experimental system. The specific aims of this project will be to (1) analyze the embryonic development of the circular nervous system; (2) identify molecular markers for the adult sea urchin nervous system; (3) determine expression patterns of homeobox-containing genes in development of the nervous system; (4) analyze functional roles of homeobox genes in patterning a radial nervous system. An evolutionary perspective on development offers an alternate way to understand how developmental programs work, what constraints on change exist, and how universal are genes that govern patterning. An understanding of these basic developmental regulatory mechanisms is vital to understanding congenital defects in human development and possible approaches to their prevention.